Sadly I couldn't find my number one so I pick my number 2 and sadly that wont make it even in 2016...
While FC is working on the new Engine and NPE a lot of which comes after the NPE is highly outdated. I guess the main focus of the Engine and NPE is attracting new players and subscribers. Whom you loose after the NPE if not earlier…
In the NPE I miss a shooting range testing area, where every class is able to test any kind of weapon and learn about specials, lock-outs and cool downs. A lot of the graphics is pimped with the introduction when the new Engine goes live but a lot is still very old and not well supported.
So my first pick would be the update of the most used assets, textures, meshes and weapon revamping (effects etc) while upgrading the existing instances with more shinny from the new engine.
Next to that, reintroduce the empty bullet shell weapon effect (lowest row middle picture - see link) for ranged weapons
Not to forget the following pictures:
http://www.ao-universe.com/forum/viewtopic.php?t=5340